This invention relates to the formation of pellets for use in a melting furnace and, more particularly, to the formation of pellets for use in a glass melting furnace.
Seng, U.S. Pat. No. 4,212,613, discloses the formation of batch material into pellets by adding the batch material and liquid, such as water, to a rotary pelletizer. Rotation of the pelletizer mixes the batch material with the liquid such that the batch material added to the pelletizer evolves from a batch appearance to non-adhering discernible nuclei or seeds which gradually grow as they are moved around the pelletizer by the pelletizer's motion. The finished pellets may be placed directly into a melter or supplied to a heat exchanger through which hot gases from either the melter or an external source are passed. Preferably, the pellets are dried and preheated in accordance with the teachings of Hohman, Seng, Henry and Propster in U.S. Pat. Nos. 4,248,615 and 4,248,616. The preheating of the pellets provides an energy savings since less energy is required to melt the batch. In addition, the hot exhaust gases from the furnace contain useful batch particulates that are imparted to the batch, thereby allowing recovery of this material which would otherwise be expelled into the atmosphere.
The components of the batch material supplied to the pelletizer tend to segregate during transportation thereto. This segregation is not harmful to the operation of a glass furnace, since the components of the pellets average out over a period of time. However, the short variations in the batch components affect the pellet forming ability of the batch material, and hence the size of the finished pellets. Certain glass batches have particle sizes that vary widely, thus resulting in greater segregation and variation of the batch than for other batches. In addition, the feed rate of the batch to the pelletizer may vary. Therefore, it is necessary to accurately predict the size of the pellets being formed within the pelletizer so that the water being supplied to the pelletizer can be controlled to respond to variations in the composition of the batch and/or the rate of feed of the batch, thereby producing pellets within the desired tolerance. As is known in the art, increasing the amount of water supplied to the pelletizer increases the size of the finished pellets; whereas, decreasing the amount of water reduces the size of the finished pellets.
The pellets should be uniform within a prescribed tolerance. Generally, pellets having a nominal diameter of one-half inch, with a range of three-eighths to five-eighths inch, have been found to be the optimum size for obtaining maximum heat transfer from the hot combustion gases to the pellets. If the pellets are too small, they excessively restrict the flow of gases through the preheater; whereas, if the pellets are too large, the surface to weight ratio is reduced which results in less heat being transferred to the pellets. Furthermore, the large pellets may have moisture trapped therein which may cause them to explode when the moisture turns to steam.
A rotary pelletizer can be divided into contiguous sections with each formative stage of the pellets pertaining to a respective section of the pelletizer. Generally, the lighter pellets, which are in the earliest formative stages, will be propelled in the widest rotational path and will follow an elliptical path with the widest circular orbit. As the pellets become heavier, the motion or path of the pellets will become more elliptical, since the heavier pellets will experience a deceleration sooner than the lighter pellets. Accordingly, it is possible to identify the formative stages, as well as the relative sizes of the pellets on a cross section of the pelletizer, by identifying the appropriate sector in that cross section.
Seng, U.S. Pat. No. 4,212,613, discloses the use of a pelletizer having a rotating disc, a sensor for detecting a variation in the batch level and means operatively connected to the sensor for varying the amount of water supplied to the pelletizer in response to the sensed variation in batch level. The sensor is positioned generally at an upward portion of the disc where it is contacted during the pelletization process by an upwardly moving stream of batch material prior to the formation of non-adhering discernible pellet seeds. The sensor consists of a paddle that is pivotally connected so that the paddle is free to move when contacted by the material on the disc of the pelletizer. The arm of the paddle is normally held against a switch by a spring to keep the switch in the open position. When the depth of the material reaches a predetermined level, the paddle is contacted and moved away from the switch, thereby allowing it to close and, in turn, close a valve to reduce the amount of liquid provided to the pelletizer.
Accordingly, it is apparent that the sensor of Seng provides only a single output which indicates whether or not the material on the disc is above or below a predetermined level. However, in many control systems it is desirable to provide a signal that is directly proportional to the level or depth of the material on the disc to proportionately control the rate of supply of liquid or batch material to the pelletizer.
Therefore, it is an object of this invention to provide an apparatus for determining the level or depth of material on the disc of the pelletizer and for providing a signal indicative thereof.